1. Field of the Invention
The present invention relates to a leak diagnosis system for an evaporative emission control system applied to an internal combustion engine, and more particularly to a leak diagnosis system which diagnoses a leakage of evaporative fuel from an evaporative emission control system for an internal combustion engine of an automotive vehicle.
2. Description of the Prior Art
A variety of evaporative emission control systems for automotive vehicles have been proposed and practically used. A typical evaporative emission control system is provided with a charcoal canister for preventing evaporative fuel in a fuel tank from being purged into the atmosphere. In this emission control system the charcoal canister adsorbs evaporative fuel from the fuel tank and purged into the engine by utilizing the negative pressure generated by the internal combustion engine.
However, such an emission control system has a possibility that evaporative fuel is purged into the atmosphere if a leakage hole is generated at an evaporative fuel conduit or if an unsealed portion is generated at a connecting portion of the evaporative fuel conduit. If such an inferiority is generated in the evaporative emission control system, it is impossible that the evaporative emission control system is sufficiently operated. In order to prevent such troubles, EPA (Environmental Protection Agency) and CARB (California Air Resources Board) have proposed a leakage diagnosis system and a practical method thereof. Such a leak diagnosis system proposed by such organizations is shown in FIG. 24 and a flowchart for controlling the diagnosis system is shown in FIG. 25. As shown in FIG. 24, an end of an evaporative fuel conduit 102a is connected to a fuel tank 101, and the other end of the evaporative fuel conduit 102a is connected to a charcoal canister 103 adsorbing evaporative fuel. A pressure sensor 106 is disposed in the way of the evaporative fuel conduit 102a in order to detect a pressure in the evaporative fuel conduit 102a. The signal from the pressure sensor 106 is inputted into an engine control unit 150 such that a PCS (pressure changing speed) in the evaporative fuel conduit 102a is obtained. The engine control unit 150 has previously stored a decision value for diagnosing a leakage of evaporative fuel and implements the diagnosis by comparing the PCS with the decision value. An end of an evaporative fuel conduit 102b is connected to the charcoal canister 103, and the other end of the evaporative fuel conduit 102b is connected to an intake side of the internal combustion engine. A purge-cut valve 104 is disposed in the way of the evaporative fuel conduit 102b. The evaporative fuel adsorbed in the charcoal canister 103 is purged into the internal combustion engine during an engine operating condition upon opening the purge-cut valve 104. A drain-cut valve 105 is connected to a bottom of the charcoal canister 103 so as to cut the communication between the charcoal canister 103 and the atmosphere during the leak diagnosis.
Such a conventional leak diagnosis system implements the leak diagnosis according to the flow chart of FIG. 25 as follows.
In a step S110 the purge-cut valve 104 is opened during a predetermined engine operating condition, so that the evaporative fuel adsorbed in the charcoal canister 103 is purged into the engine due to intake negative pressure generated in the engine.
In a step S120 the drain-cut valve 105 is closed such that the negative pressure of the engine is applied to the evaporative emission control system.
In a step S130 the engine control unit 150 detects the pressure in the evaporative emission control system during a sucking operation as mentioned above, on the basis of the signal of the pressure sensor 106. It will be understood that a time period from the start of the suction by the engine until a predetermined negative pressure may be used as a signal instead of the PCS. That is, in the step S130 it is judged whether the pressure reaches a predetermined pressure or not, or whether a predetermined time has elapsed from the start of the sucking operation.
In a step S140, the purge-cut valve 4 is closed.
In a step S150, the engine control unit 150 monitors the pressure change in the inner space on the basis of the signal of the pressure sensor 6 and detects a pressure changing speed (PCS) or pressure buildup speed toward the atmospheric pressure.
In a step S160, it is judged whether the detected pressure buildup speed is larger than the decision value or not. When the judgment in the step S160 is "YES", the program proceeds to a step S170 wherein it is judged that the system is in a leak condition. When the judgment in the step S160 is "NO", the program proceeds to a step S180 wherein it is judged that the system is in a no-leak condition.
Following the operation of the step S170 or S180, the program proceeds to a step S190 wherein the drain-cut valve 105 is opened.
However, the PCS (pressure changing speed) in the evaporative emission control system takes various values due to the fuel condition in the evaporative emission control system, such as the change of the inner space volume which changes according to the remaining amount of fuel in the fuel tank, as shown in FIG. 26 and the change of the generating amount of fuel vapor which changes according to the fuel temperature, as shown in FIG. 27. Accordingly, it is desired to accurately diagnose the leakage condition of the evaporative emission control system without being effected by the fuel condition in the evaporative emission control system such as the remaining amount of the fuel in the fuel tank and the fuel temperature.